The invention relates to a method for the material removal to a predetermined removal depth from a workpiece by means of a laser beam consisting of one or more sub-beams, each of the latter having a defined beam axis, whereby the axis of the laser beam or the individual axes of the sub-beams are guided along a removal line at a predetermined travelling speed and the laser beam has a predetermined spatial energy flow density that defines a Poynting vector S with a value I0f(x) and a direction s, the spatial energy flow density creating a removal face with an apex formed by the leading part of the removal face in the removal direction and said face creating a removal edge. The invention furthermore relates to a device for carrying out the method.
Laser cutting is an established method for material removal through fusion at which the created removal depth encompasses the entire thickness of the workpiece. When cutting metals, the removal face is molten and is also referred to as cutting face. Among the laser-aided manufacturing methods, it takes the leading position in industrial applications. From the perspective of the user, the increase in productivity of the method and an improved quality are constant requests.
It is known that high-performance CO2 lasers (10μ emitters) with a wavelength of the radiation of approx. 10 μm and with a laser power of 1-6 kW are used industrially for laser cutting (in the field of macro applications for sheet metal in a range from 1 mm to 30 mm). In addition, fiber lasers and disc lasers (1μ emitters) with a wavelength of the radiation of approx. 1 μm and with a laser power of 1-8 kW are used for laser cutting. In particular these radiation sources offer economic advantages and are, therefore, used increasingly. However, it is becoming apparent that the cut quality at the workpiece is dependent, for example, on the radiation source in use (fiber laser, disc laser 1 μ-emitter, gas laser 10μ emitter) and, for example, on the sheet thickness to be cut and the feed rate. For this reason, the following significant quality features must be achieved reliably during laser fusion cutting:
Productivity of the Process:
Shorter machining times and the high-quality removal or separation of greater material thicknesses are fundamental demands. For this reason, increasingly greater laser powers and systems with high-quality drives are introduced in manufacturing. The development aims at expanding the technical limits of process control.
Quality of the Cut Edge:
In addition to roughness and adhering burrs as well as the formation of oxide coatings, evenness and right angles are significant quality features for the cut edge. The process chain cutting-welding is an example by which the significance of the quality of the cut edge for preparing the joining gap can be recognized. To be able to generate with a laser slim welding seams that require no post-processing by grinding or dressing, a cut of the components to be joined with plane, right-angled as well as smooth burr- and oxide free cut edges is desired. The following sub-points should, therefore, be considered:                With an increasing sheet metal thickness, the cut edge exhibits increasingly rough gouges, which appear in particular in the lower part of the cut edge (or abrasion kerf, respectively). This problem is even worse with greater sheet thicknesses.        In particular with low or high feed rates the melt is not fully removed from the bottom edge. The attached and then solidifying melt forms the undesired burr. The mechanisms of how such burrs are generated are understood only in part.        The formation of cracks and pores in the weld seam can be caused by oxidized joining edges, such as the ones that occur with flame cutting. For this reason, fusion cutting with an inert cutting gas is employed to obtain oxide-free cut edges.        
The known techniques for cutting metals using laser radiation are divided through the involved mechanisms for introducing the cutting energy into                Laser beam cutting with reactive cutting gas stream        Laser beam cutting with inert cutting gas stream.        
In laser beam cutting with a reactive cutting gas stream (e.g., oxygen, compressed air), the laser beam and an exothermic chemical reaction together provide the cutting energy. Techniques for laser beam cutting with a reactive cutting gas stream are further differentiated by whether the laser beam acts dominantly in the cut gap (laser beam flame cutting) or is additionally radiated onto the upper side of the sheet metal (burn-up-stabilized laser beam flame cutting).
In laser beam cutting with an inert cutting gas stream (e.g., nitrogen), the laser beam generates the cutting energy. Laser beam cutting with an inert cutting gas stream is further differentiated by the varying mechanisms for accelerating/expelling the melt. In addition to the effect of the cutting gas stream, the evaporation of molten materials may occur and accelerate the melt. The driving effect increases with an increase in the feed rate due to the evaporation.
Three process variants are differentiated in laser cutting with an inert cutting gas stream:
(1) Laser Beam Fusion Cutting:
The temperature at the surface of the melt remains below the evaporation temperature and the melt is expelled by the cutting gas stream only. This process variant is used industrially for fine, medium and thick metal sheets. The melt flows out primarily at the apex of the cutting face—in front of the laser beam axis. Gouge and burr formation constitute limitations to the quality.
(2) Fast Cutting:
The evaporation temperature is exceeded in the lower part of the cutting face and the expelling effects based on the cutting gas and of the evaporating material are comparable. The melt flows out primarily in the front area of the cutting face, to the right and left of the laser axis. This process variant can be used for fine and medium sheet metal. Gouge and burr formation constitute limitations to the quality.
(3) High-Speed Cutting:
The evaporation temperature is exceeded almost on the entire cutting face. The expelling effect due to evaporation is dominant. The melt flows around the laser beam axis and closes a portion of the cutting gap in the wake of the laser beam and is expelled there through the effects of the cutting gas. This process variant is used for fine sheet metal.